Experiments have been designed to test the hypothesis that the two cellular repair activities observed after exposure of mammalian cells to sparsely ionizing radiations (recovery from sublethal damage (SLD) as observed in split-dose experiments, and repair of potentially lethal damage (PLD) observed after various postirradiation treatments of cells) might have a class of molecular lesions in common, the repair and/or fixation of which causes the effects observed at the cellular level, even in different experimental protocols. Also, the hypothesis will be tested that the variation in cell survival, observed throughout the cell cycle, might be the result of repair and/or fixation of PLD mediated by molecular activities initiated at certain borders of the cell cycle. To test the first hypothesis, experiments will be performed with plateau phase V79 and 10 T 1/2-cells on the expresion of PLD as caused by Beta-araA and Beta-araC, specific inhibitors of the DNA polymerases Alpha and Beta. In other cell systems, these treatments mainly affect the shoulder width (Dq) of the survival curve. The repair kinetics for recovery from sublethal damage and repair of PLD, as measured by treatment with inhibitors various times after irradiation, will be compared. Further, the spectrum of potentially lethal lesions affected by these treatments will be compared to that of potentially lethal lesions affected after postirradiation treatment with hypertonic solutions using as criteria (a) the repair time constants observed, and (b) the effect of each treatment on the parameters of the survival curve. By screening also the effects on the repair of PLD of specific inhibitors of DNA polymerase Alpha, such as aphidicolin, the involvement of polymerase Alpha or Beta in these repair processes will be tested. To test the second hypothesis, the effects of caffeine in expressing potentially lethal damage will be studied throughout the cell cycle, and the survival curves obtained at concentrations and treatment times giving maximal effect will be compared with those obtained at the borders of maximal sensitivity. The reparability of PLD induced in one phase of the cell cycle will also be tested in subsequent phases by incubating cells under appropriate conditions as they progress through the cycle to test whether G1/S-border and mitosis are the stages in which unrepaired, potentially lethal damage becomes fixed.